Field of the Invention
The present invention relates to a method of reducing output degradation of a gas sensor, and particularly relates to reduction of output degradation attributable to adhesion of a gas component to an electrode.
Description of the Background Art
Gas sensors that sense a predetermined gas component in a measurement gas such as an exhaust gas, for example, to determine its concentration come in various types such as a semiconductor gas sensor, a catalytic combustion gas sensor, an oxygen-concentration difference sensing gas sensor, a limiting current gas sensor, and a mixed-potential gas sensor. Some of these gas sensors, which are obtained by providing electrodes containing a noble metal or a metal oxide as its main constituent to a sensor element mainly made of ceramic being a solid electrolyte such as zirconia, are widely known.
As is also well-known, in a gas sensor which includes a sensor element mainly made of ceramic such as zirconia, a gas component in a measurement gas or a poisoning substance is adhered to the surface of the electrode due to long-term use, or a constituent material of the electrode is sintered due to exposure of the electrode to a high-temperature atmosphere, so that an output value may vary in spite that a concentration of a gas component to be measured in a measurement gas is constant.
Among the above-stated causes for output change of a gas sensor, adhesion of a poisoning substance and sintering of a constituting material of the electrode are irreversible phenomena, and it is considered to be difficult to cope directly with change in output value due to electrode deterioration (irreversible deterioration) caused by those phenomena.
On the other hand, it is possible to cope with output change due to adhesion (adsorption) of a gas component in a measurement gas to the surface of the electrode, by carrying out a predetermined recovery process and removing the adsorbed gas component. That is, such output change is caused due to electrode deterioration (reversible deterioration) caused by a reversible factor. With regard to a gas sensor subjected to such reversible deterioration, execution of a recovery process would allow an original (initial) output value to be re-attained, or would allow an output value as close to the original output value as possible to be obtained.
Examples of the foregoing recovery process include an electrical process (for example, refer to Japanese Patent Application Laid-Open No. 6-265522 (1994) and Japanese Patent No. 3855979), and a heating process (for example, refer to Japanese Patent Application Laid-Open No. 11-326266 (1999)).
The electrical process is a method for recovery output by alternately applying positive and negative potentials between electrodes that are paired through a solid electrolyte, so as to refine the electrode or to desorb an absorbed substance.
On the other hand, the heating process is a method for recovery output with exposure of an adsorbed substance or a poisoning substance to a high temperature to desorb or burn (oxidize) the substance.
It is already publicly known that, in an engine system including an oxidation catalyst halfway through an exhaust pipe connected with an engine, an exhaust gas externally discharged through the exhaust pipe contains a large fraction of a hydrocarbon gas at what is called cold start before the oxidation catalyst activates (refer to “NSC/SDPF System as Sustainable Solution for EU6b and Up-coming Legislation”, L. Mussmann et al, 23rd Aachen Colloquium Automobile and Engine Technology 2014, P.1025, for example).
The recovery process can be performed at an optional timing. Thus, at any time when the recovery process becomes necessary in actual use of the gas sensor, the output recovery can be performed. However, in this case, the output of the gas sensor is unavailable during the recovery process.
The inventor of the present invention has researched improvement of the measurement accuracy of a gas sensor to eventually find that adsorption of a gas component in a measurement gas to the surface of an electrode is likely to occur at lower operating temperature of a sensor element. Typically, a gas sensor provided to an engine system and configured to sense, as a measurement gas, an exhaust gas from an engine is driven for use at “key-on” (start) of the engine. Totally considering the above-described finding by the inventor of the present invention and the publicly known fact disclosed in Mussmann et al. that an exhaust gas at cold start contains a larger fraction of a hydrocarbon gas, the gas sensor has been conventionally used while a hydrocarbon gas component is certainly adsorbed to the electrode, in other words, while the measurement accuracy is degraded.